Vanxa



Patented J une 10, 1919.

Fly.

F. THORNTON. 1R., AND 0. A. COLBY.

ELECTRIC FURNAC.

APvucAnuN mm ocr.24.19|1.

INVENTOR ATTORNEY UNITED STATES PATENT OFFICE.

FRANK THORNTON, JR., F PITTSBURGH. AND ORA A. COLBY, 0F LARIMER,PENNSYL- VARIA, ASSIGNORS T0 WESTNGHUSE ELECTRIC & MANUFACTURINGCOMPANY, A

CORPORATION QF YENNSYLVANIA.

ELECTRIC FURNACE.

' To all 'whom it may concern.' i

Be it known that we,FnANx THORNTON,

. Jr.z a citizen of the United States, and a consequently, fails toheat. u A tetes a, removal of the consumed resistor maresident ofPittsburgh, in the county of Alleghen and State of Penns lvania, and ORAA. nur, a citizen of the nited States, and a' resident of Larimer, inthe county of Westmoreland and State of Pennsylvania, have invented anew and useful Improvenient in Electric Furnaces, of which the followingis a specification.

This invention relatesto electric furnaces and, in particular, tohigh-temperature elcctric furnaces for heating and treating metals andalso for other heating applications.

In working with those types of electricresistancerfurnaces in which theresistor conl tains `granular material, we have found that therateofcombustion of the granular resistor, material increases during1 theoperation ofthe furnace and, in a comparatively short time, becomes sogreat as to cause a rapid inclease in resistance, and the resistor,

This news-:si-

terial and a renewal of the same which interferes with the operation ofthe furnace and is also expensive.

We have found also that the limits of temperatures for a furnace usinggranular carbonaceous material, such as granulated aphite, colto, andcharcoal, as a resistor f 1s around .900 C., because, when a temJperature of about 1000 C. is maintained on the surface of the resistorwhich coirtitutes the hearth of the heating chamber, the teniperature inthe interior of the resistor is around 2000 C., which exceeds the safelimit of the refractory material of the furnace. This condition is dueto the fact that carbon has a negative temperature-resistanoecoefficient and thc .current through a m'ass of any form of carbon or,graphite tends to concentrato in the zone of highest y temperaturewhich, in this case, is in 4=. part of t e resistor away fromthe heatingchamber, since the portion of the resistor exposed in the heatingchamber will be cooler than other portions of the resistor. Accordingly.an object of this invention is to provide an electr1c-resistanco furnacein which high tcm eratures are obtainable without a rate of eterimatonthat will rtm1?? *he fumes@ Specification of Letters Patent` Iatntea .june 10, 1919.

Application lled October 24, 1917. Serial No. 198,275; i i .i

too expensive to operate with ,commercial success. p l y Another objectof our inventionv is to -provide a furnace which has a preheating chambel' in which the articles to be treatedinay be gradually brought up toa. moderately hightemperature before they are introduced into the mainheating chamber ffth'e furnace. By arranging the preheatin chamber inthe furnace structure near t e main heating chamber, with the resistorbetween the two chambers, a path is provided foil the escape of heatfrom the lower part of the resistor, thereby equalizing the temperatureof the resistor and its lining, so that higher temperatures may besafely ymaintained in the heating chamber.

In that type of furnace in which the heat is generated only in the Hoorof the hearth or heating chamber, the side walls and roof obtain theirheat only by direct radiation, and pieces of metal that are laid closelyover the surface of the hearth will interfere with the radiation of heatdirectly in proportion to the amount of hearth surface that they cover.lIn a continuous] operated furnace, this condition prevents t c roof .ofthe furnace from reaching a temperature equal to that of the hearth orthe metal, and such a furnace can, therefore, only bo operated at a verylow rate of production. In order to avoid this condition, it is anobject of this invention to provide an electric-furnace resistor ofcarborundum forming the top and bottoni walls of the heating chamber. sothat heat may be generated at every point in the roof and floor of theheating chamber. A high temperature is thereby maintained oualiradiating surfaces in the chamber, and heat will be fed into the workthrough all its exposed surtions of another electric-resistance furnaceembodying our invention; and Fig. 4 1s a sectional View, taken on theline II-III of Fi 3.

5 Rerringto Figs. 1 and 2, the furnace construction comprises a suitableiron framework and exterior shell 10, that carries the vertical walls 11and a bottom wall 12 of refractory material, the latter supporting arefractory brick base structure 13. A center wall 14 is spaced from theside walls 11 to form hoppers 15 into which granular 'material, such asgraphite or coke, `may be charged -for a purpose to be described. Thehoppers 15 extend downwardly into the fur-fV nace structure and in eachof them is a solid terminal electrode 16 vof graphite or amorphouscarbon for leading current to the resister. The electrodes 16 arelocated in the bottoms of the hoppers 15 and, at their lower ends, areconnected to metal plates 17 which extend downwardly throu h the ioor ofthe furnace and form `termina s to which the supply mainsare connected.

The main heating chamber 18 of the furnace is open, at one end, andotherwise inclosed by top and bottom walls 19 and 20 separated by thespacers 21 which form the si e walls of the heating chamber. The

'walls 19 and 20 consist of carborunduin or silicon carbid bricks orslabs, and the intermediate s acers 21 may also consist of solidcarborun um blocks, but any highly refractory material will serve thepurpose, since it is not necessary that these spacers ybe of electricalconducting material.

The carborundum walls 19 and 20 of the chamber `18 constitute theresistor of the furnace, which is spaced from the up er taperingl endsof the terminal electro es 16. These spaces between the resistor and theelectrodes are filled with masses 22 of granular graphite or coke,packed to provide a medium between the electrodes and the resister thatis an electrical conductor and also a thermal insulator of greatercrosssectionthan the resistor to prevent overheatin and consumption ofthe expensive electroe 16. The masses 22 oxidizc and a-re 5 graduallyconsumed and may bc replaced by additional material introduced :1ndtamped into ythe upper ends of the hoppcrs 15.

The bottom Wall 20 forms the floor-of the hea-ting chamber 18, and theupper and lower surfaces of thc top and bottom walls 19 and 20respectively, arc covered by n lining 23 of a highlyrefractory.composition consisting of carboru nduin sand and silicate ofsoda or earborundun sand, cruciblc clay and graphite. This lining servesto retard the leakage of heat from the resistor blocks 19 and 20, and,since the resistance of carborundum changes slowly at high temperatures, the top and bottom walls of the chamlber, after a few hours runand heat intcrchange, will have approximately the same resistance andeventually will have approximately the same temperature. Thelresistorinclosin -the main heatingchamber 18 rests on the re-brick base 13, andwithin the latter is inclosed `a prehealting chamber 24 directly beneaththe heating chamber l8r and separated from the latter by thelowerportion of the refractor lini 23 and the refractory wa1l25. T e pre eat'chamber 24 is thus located so that it will erive heat from the lowercarborundum wall 20, which loses art of its heat throu h the lining 23and t e wall 25. Cold arties, such as cold ieces of metal, would bedamaged if intro- 80 duced directlg' into the,A high-temperature Yheating cham er 18 and they are therefore, first heated to a moderatelyhi htempe'rature in the preheati 'chamber 24, 'after which they areintro uced into the high/35 temperature chamber 18. 1

The silicon carbid or carborundum is very suitablefor use as avresistor4since -it resists fusion and oxidation up to an ext'remelydiightemperature, is ri -d, of good heat conductivity and is availab einvarious molded shapes.\ Although carborundum has a larger negativetemperature-resistance co-v efficient than carbon, it is superior tocarbon as a resist-or for high-temperature `fur-- )laces because of thecombustion which takes place on the exposed surface of graphite orcarbon blocks at high,tempe`ratures, and the diiculty of repairor'replacement of burned y portions. Moreover, it 1s possible to utilize100 the negative temperature-resistance coeicient characteristic ofcarborundum 'as 'a basis for controllin the furnace. The lack ofcombustion of t e carborundum resistor material results in there beingan oxidizing atmosphere within the furnace, but a nonoxidizingatmosphere `may beobtained byA "spreading granulatedcharcoal or Vcarbonover the surface ofythe resistor near the door where it is burned by theairthat enters. 4With a furnace constructed according to .thisinvention, it has been -pOSSible to obtaincori- 'v tinuous operation attemperatures as'4 high as 1250 C. without injury to any part of? thefurnace and"the' temperature dp 11b through the silicon carbid orcarborundum is' very much less than that which has been observed througha granular resistance bed, uml the danger of damaging the lining is verymuch reduced. i

IdFigs. v3 and 4, we have illustrated n furnace construction embed ingfeatures oi the' furnace illustrated in figs- 1 and 2-but in which onlythe floor 26 of the heating chamber is built up of carborundum orsilicon carbid bricks. "lhesc bricks constitute the resistor of thefurnace and replace the granular resistance bed illustratedanddescribediu our copending application` In the particular formillustrated, there are 13o insulating conducting three parallel spacedrows 261 of bricks, iid on edge end to end, and between the p titillerrow and the twoouter rows are rows 2C2`=0fbric`ks which break jointswith the center and outer ro'ws and are laid on their wider sides. Metalbars placed in the heating chamber 181 will then be supported atintervals" onlyand will be` spaced from the intermediate rrows 262. Thefioor 26 is inclosedat the bottom andsides by a highly refractorycarbonaeeous lining 2T similaito the lining 23, the lining 27 beingsupported by the fire-brick wall 28 that forms the roof of thepreheating chamber 241. The ends of "the resistor formed by the floor 26are connected to suitable terminal electrodes (not shown), such as `theelectrodes 1G of lf 1, by the granular masses of thermal Same manner asthose illustrated in Fig. 1. Temperatures, up to about 1200o C., areobtainedvin this furnace without difficulty and the furnace is adaptedto be used with success for hardening tools and dies and for firingporcelain. A non-oxidizing atmosphere may be obtained within the furnaceby spreading granular charcoal or carbon over the floor 26 near the doorwhere it is burned by the entering air.

ile We have shown and described our invention in detail, it is to beunderstood that numerous changes and adaptations may be made withoutdeparting from the spirit of the invention as defined in the appendedclaims.

We claim as our invention:

i 1. In an electric furnace, the combination with a heating chamber, ofa resistor adjacent the same comprising blocks of refractory electricalconductin7 material in contact with one another and forming the top andbottom walls of said chamber, terminal electrodes, and a mass ofthermalinsulating and electrical-conducting granular material interposedbetween each of the electrodes and the resistor to prevent overheatingof the electrodes.

In an electric furnace, the combination with a heating chamber, of aresistor comprising blocks of refractory electricalconducting materialin contact with one another and forming certain walls of said. chamber,hoppers on opposite sides 'of said chamber, terminal electrodes in saidhop ers, and a mass of thermal-insulating an electrical-conductingmaterial between each of said electrodes and the resistor and of greatercross-section than the resistor to prevent overheating of theelectrodes.

3. In a high-temperature electric furnace, the combination with aheating chamber having walls formed of blocks of refractoryelectrival-conducting material constituting the resistor of the furnace,of hoppers on opposite sides of said Chamber, solid material 221, inthe`` 'faire resistorof the furnace.

tibll carbonaceous terminal electrodes in Said hoppers, and aniass ofthermal-insulating and electrical-condoeting material between l euch ofsaid electrodes and the resistor, said masses of material being granularand of 70 greater cross-section than the resistor to prevent overheatingand consumption of the l electrodes.

4. An electric furnace comprising a heating chamber having an open end,top and 15 'bottom walls formed of solid refracto electrical-conducting`material which constltutes the resistor of the furnace, and side" wallsof refractory material for separating the top and bottom walls.

5. A high-teuriieratl1re electric furnace comprising a heating chamberhaving walls formed of solid refractory cle'ctrical-conf ductingi'uaterial, said material constituting terminal elec- 86 and a mass ofthermalinsulating`a`nd" electrical-conducting material between saidelectrodes and theopposite outer sides of said chamber.

G. A high-temperature .electric Vfurnace 90 comprising a heatingch'amber having topand bottom walls formed of blocks uflrefracboryelectrical-conducting material con`- stituting the resistor of thefurnace.

7. A higb-temperature electric heating furnace comprising a resistorconsisting of blocks of electrical-conducting refractory materialinclosim;r a space which constitutes the heating chamber of the furnace,terminal electrodes, and a mass of thermal-insulating andelectrical-conducting material between each of said electrodes and saidresistor, said masses of conducting material being granular and ofgreater cross-Section than the resistor to prevent overheating andconsumption of said electrodes. Y

8. An electric furnace comprising a heating chamber, a preheatingchamber, and a resistor for supplying heat to both of said chambers.

9. An electric heating furnace comprising a heating chamber, apreheatingchamber and a resistor between said chambers and arranged tosupply a higher degree of heat to said heating chamber than' to Said 115preheating chamber.

10. `An electric furnace comprising a heating chamber` a preheatingchamber, and a resistor having a part disposed between said chambers andcloser to the heating chamber than to the preheating chamber so as tosupply a higher degree of heat to the former than to the latter.

11. An electric furnace comprising a heating chamber, a preheatingchamber, and a resistor having apart disposed between said chambers toconstitute a wall ,for the heating chamber and separated by a layer ofmaterial from the prcheating chamber` ,ll-2. .An electric furnacecomprising a heating chamber and a preheating chamber dis-I poscd oneabove the other, a resistor having a part disposed between said chambersto constitute a wall for thc heating chamber and separated by alayer-.of material from the preheating chamber, terminal electrodes onopposite sides of said chambers, and -a mass of electrical conducting'material between each electrode and said resistor.

13. An electric furnace comprisin a heating chamber, a preheatingchamoerisposed directl `below said heating chamber, a resistor, aving a partconstituting 'the floor 1 .of said heating chamber, a layer of materialof low thermal and electrical conductivity between said` resistor andsaid preheating chamber, hoppers at onnosite sides of said resistor,terminal electrodes in said hpprs,

and a mass of electrical-conducting, thermal-insulating granularmaterial between cach electrode and the resistor to prevent overheatingof said electrodes.

14. An electric-resistance furnace having a heating chamber providedwith top and bottom walls comprising material that has la largernegative temperatureresistance coefficient than carbon, said wallsconstituting the resistor of the furnace.

15. An electrical-resistance furnace having a heating chamber providedwith a top wall of carbid that has a negative temperature-resistancecoefficient, said wall constitutin g the resistor of the furnace.

16. An electric-resistance furnace comprising a heatinfr chamberprovided with top and bottom walls of silicon carbid that has a large neative temperature-resistance coecient, saiv walls constituting theresistor of the furnace.

17. A high-temperature electric-resistance furnace comprising top andbottom walls consisting of silicon carbid that has a large negativetemperaturefresistance coeilicient and side walls which inclose a s acethat constitutes the heating chamber o the fur-,

nace. i

18. An electric-resistance furnace com prising a resistor consisting ofblocks of silicon carbid s aced apart to form the heatin chamber o thefurnace.

n testimony whereof, lwe have hereunto subscribed our names 1917.

FRANK THORNTOMJB; ORA A. coLBY. Y

`this 22nd day 0f 0613-,

